Lattice matching layer for use in a multilayer  substrate structure

ABSTRACT

A lattice matching layer for use in a multilayer substrate structure comprises a lattice matching layer. The lattice matching layer includes a first chemical element and a second chemical element. Each of the first and second chemical elements has a hexagonal close-packed structure at room temperature that transforms to a body-centered cubic structure at an α-β phase transition temperature higher than the room temperature. The hexagonal close-packed structure of the first chemical element has a first lattice parameter. The hexagonal close-packed structure of the second chemical element has a second lattice parameter. The second chemical element is miscible with the first chemical element to form an alloy with a hexagonal close-packed structure at the room temperature. A lattice constant of the alloy is approximately equal to a lattice constant of a member of group III-V compound semiconductors.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of provisional applications 61/662,918,filed on Jun. 22, 2012 and 61/659,944, filed on Jun. 14, 2012, thecontents of which are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The example embodiments of the present invention generally pertain tosemiconductor materials, methods, and devices, and more particularly toa multilayer substrate structure for epitaxial growth of group III-Vcompound semiconductors.

BACKGROUND

Group III-V compound semiconductor, such as gallium nitride (GaN),gallium arsenide (GaAs), indium nitride (InN), aluminum nitride (AlN)and gallium phosphide (GaP), are widely used in the manufacture ofelectronic devices, such as microwave frequency integrated circuits,light-emitting diodes, laser diodes, solar cells, high-power andhigh-frequency electronics, and opto-electronic devices. To improvethroughput and reduce manufacturing cost it is desired to increase size(e.g., diameter) of substrates. Because growing III-V compoundsemiconductors of large size is very expensive a great number of foreignmaterials including metals, metal oxides, metal nitrides as well assemiconductors, such as silicon carbide (SiC), sapphire and silicon, arecommonly used as substrates for epitaxial growth of III-V compoundsemiconductors.

However, epitaxy growth of group III-V compound semiconductors (e.g.,GaN) on substrates (e.g., sapphire) poses many challenges on crystallinequality (e.g., grain boundaries, dislocations and other extendeddefects, and point defects) of the epitaxial layers due to latticemismatch and coefficient of thermal expansion mismatch between the GaNlayer and the underlying substrate, a foreign material. Differences inthe coefficient of thermal expansion between the GaN layer and theunderlying substrate result in large curvatures across the wafer,resulting during and post processing upon returning to room temperature,and the large mismatch in lattice constants leads to a high dislocationdensity, unwanted strain and defects propagating into the epitaxial GaNlayer. In order to cope with these problems, stress relaxationstrategies are employed, such as growing buffer layers between the GaNlayer and the sapphire substrate, or counter balancing compressive andtensile strain by alternating appropriate material layers. However, byadding the buffer layer or stress relieving layers, the dislocationdensity may remain high and the manufacturing cost and complexityincreases significantly because of the use of the same depositiontechniques involved in growing the active device layers.

BRIEF SUMMARY

According to one exemplary embodiment of the present invention, amultilayer substrate structure comprises a lattice matching layer. Thelattice matching layer includes a first chemical element and a secondchemical element. Each of the first and second chemical elements has ahexagonal close-packed structure at room temperature that transforms toa body-centered cubic structure at an α-β phase transition temperaturehigher than the room temperature. The hexagonal close-packed structureof the first chemical element has a first lattice parameter. Thehexagonal close-packed structure of the second chemical element has asecond lattice parameter. The second chemical element is miscible withthe first chemical element to form an alloy with a hexagonalclose-packed structure at room temperature. A lattice constant of thealloy is approximately equal to a lattice constant of a member of groupIII-V compound semiconductors.

According to one exemplary embodiment of the present invention, a methodof fabricating a multilayer substrate structure comprises growing alattice matching layer. The lattice matching layer includes a firstchemical element and a second chemical element. Each of the first andsecond chemical elements has a hexagonal close-packed structure at roomtemperature that transforms to a body-centered cubic structure at an α-βphase transition temperature higher than room temperature. The hexagonalclose-packed structure of the first chemical element has a first latticeparameter. The hexagonal close-packed structure of the second chemicalelement has a second lattice parameter. The second chemical element ismiscible with the first chemical element to form an alloy with ahexagonal close-packed structure at room temperature. A lattice constantof the alloy is approximately equal to a lattice constant of a member ofgroup III-V compound semiconductors.

According to one exemplary embodiment of the present invention, amultilayer substrate structure comprises a lattice matching layer. Thelattice matching layer includes a first chemical element and a secondchemical element. Each of the first and second chemical elements has ahexagonal close-packed structure at room temperature that transforms toa body-centered cubic structure at an α-β phase transition temperaturehigher than the room temperature. The hexagonal close-packed structureof the first chemical element has a first lattice parameter. Thehexagonal close-packed structure of the second chemical element has asecond lattice parameter. The second chemical element is miscible withthe first chemical element to form an alloy with a hexagonalclose-packed structure at room temperature. A linear relation existsbetween the first and second chemical elements and their associatedlattice parameters at constant temperature to allow lattice constant ofthe alloy approximately equal to that of a group III-V compoundsemiconductor.

BRIEF DESCRIPTION OF THE DRAWING(S)

Having thus described the example embodiments of the present inventionin general terms, reference will now be made to the accompanyingdrawings, which are not necessarily drawn to scale, and wherein:

FIGS. 1A-1D illustrate cross-sectional views of exemplary multilayersubstrate structures in accordance with exemplary embodiments;

FIG. 2A illustrates a schematic of a hexagonal close-packed structure;

FIG. 2B illustrates a schematic of a unit cell showing latticeconstants;

FIG. 3 illustrates a periodic table; and

FIG. 4 illustrates a phase diagram correlation between transitiontemperature and atomic percentage of constituent elements in accordancewith an exemplary embodiment.

DETAILED DESCRIPTION

The various embodiments are described more fully with reference to theaccompanying drawings. These example embodiments are provided so thatthis disclosure will be thorough and complete, and will fully convey thescope of the invention to readers of this specification having knowledgein the technical field. Like numbers refer to like elements throughout.

FIG. 1A illustrates a cross-sectional view of an exemplary miltilayersubstrate structure 100 in accordance with an exemplary embodiment. Themiltilayer substrate structure 100 may include a substrate 102 and anepitaxial layer 104 epitaxially grown on the substrate 100. Depending onvarious applications, the substrate 102 may comprise a semiconductormaterial, a compound semiconductor material, or other type of materialsuch as a metal or a non- metal. For example, the material may comprisemolybdenum, molybdenum-copper, mullite, sapphire, graphite,aluminum-oxynitrides, silicon, silicon carbide, zinc oxides and rareearth oxides, and/or other suitable material.

The epitaxial layer 104 may include group III-V compound semiconductors,such as aluminum nitride (AlN), gallium nitride (GaN), indium galliumnitride (InGaN) and indium nitride (InN). As described above, there maybe a lattice constant mismatch between the substrate 102 and theepitaxial layer 104. To decrease or eliminate the defects resulting fromthe lattice constant mismatch, the epitaxial layer 104 growth on thesubstrate 102 may use a lattice matching layer 106 with thickness in arange of 5 nm-100 nm to accommodate the lattice constant mismatchbetween the substrate 102 and the epitaxial layer 104. The latticematching layer 106 may comprise two or more constituent elements, forexample of two constituents, a first chemical element and a secondchemical element, to form an alloy. The first chemical element ismiscible with the second chemical element in this alloy. The constituentelements may have similar crystal structures at room temperature, suchas hexagonal close-packed structure, as shown in FIG. 2A. Each of theconstituent elements may have its respective lattice constants includinglattice parameters along a-axis, b-axis and c-axis, and latticeparameters of interaxial angles α, β and γ, as shown in FIG. 2B. Inaddition to the crystal structures, the constituent elements may havesimilar chemical properties. In one embodiment, the first and secondchemical elements may both belong to group four elements (namely,titanium (Ti), zirconium (Zr), hafnium (Hf) and rutherfordium (Rf)) inperiodic table illustrated in FIG. 3. In this way, the alloy may be madefrom elements Ti and Zr, elements Ti and Hf, and elements Zr and Hf andmay have similar crystal structure to the constituent elements at roomtemperature, or by any combination of 2. The alloy may comprise a thirdchemical element or more elements which have similar crystal structuresand similar chemical properties.

A linear relation may exist between the first and second chemicalelements and their associated lattice parameters at constant temperatureto allow the lattice constant of the lattice matching layer 106 to beapproximately equal to that of the epitaxial layer 104. The molefraction in atomic percentage of the first chemical element to thesecond chemical element is P₁ to (1-P₁). The mole fraction may vary fromapplication to application, as the composition will control theresulting lattice parameter value of the alloy. In one embodiment, whenthe epitaxial layer 104 includes GaN and the alloy includes Ti mixedwith Zr, atomic percentage P_(Zr) of Zr may be greater than 75% and lessthan 90%. For example, P_(Zr) may be about 86%. It follows that atomicpercentage P_(Ti) of Ti is (1-P_(Zr)). A first lattice parameter of Zr,e.g., a-axis lattice parameter a_(Zr), is 3.23 Å. A second latticeparameter of Ti, e.g., a-axis lattice parameter a_(Ti) is 2.951 Å. As aresult, lattice constant P_(A) along a-axis of the alloy isP_(Zr)×a_(Zr)+(1-P_(Zr))×a_(Ti)=86%×3.23+14%×2.951=3.19 Å which isapproximately equal to the a-axis lattice constant P_(GaN) of hexagonalclose-packed GaN where P_(GaN)=3.189 Å. Depending on the constituentelements and other factors, the atomic percentage of the first chemicalelement to the second chemical element may be about 43% to 57% or 99% to1%.

When the epitaxial layer 104 includes different compound semiconductors(e.g., AlN, InGaN, InN and/or other group III-V compoundsemiconductors), the constituent elements of the lattice matching layer106 and/or the mole fractions of the constituent elements may beadjusted to make the lattice constant of the lattice matching layer 106accommodate that of the epitaxial layer 104. For example, when theepitaxial layer 104 comprises AlN and the constituent elements of thelattice matching layer 106 are Zr and Ti, the atomic percentage of Zrmay be adjusted to be lower than 75% and higher than 50%. In anotherembodiment using the same constituent elements, when the epitaxial layer104 comprises InGaN, the atomic percentage of Zr may be greater than90%. In addition to the material of the epitaxy layer 104, the thicknessof the epitaxial layer 104 may cause the changes of the selection of theconstituent elements as well as mole fraction of the constituentelements to achieve 100% lattice match. Despite the changes of thethickness of the epitaxial layer 104, it may be in a range of 5 nm-500nm. In other words, the thickness and the material of the epitaxiallayer 104 may determine the selection of the constituent elements andtheir mole fraction in forming the lattice matching layer 106. By usingany epitaxial techniques, such as vacuum evaporation, sputtering,molecular beam epitaxy and pulsed laser deposition, metalorganicchemical vapor deposition, atomic layer deposition and/or any othersuitable epitaxial deposition methods, the epitaxial layer 104 isepitaxially grown on the lattice matching layer 106 to transfer thecrystallographic pattern of the lattice matching layer 106 to theepitaxial layer 104. The lattice matching layer 106 may be formed on theunderlying layer, for example, the substrate 102 using one of depositiontechniques, such as vacuum evaporation, sputtering, molecular beamepitaxy and pulsed laser deposition, atmospheric chemical vapordeposition, and atomic layer deposition.

Because hexagonal close packed phase (α phase) has potential superiorityover the body centered cubic phase (β phase) for certain opto-electronicdevices and power semiconductor applications, it may be desired to growthe epitaxial layer 104 in α phase to achieve similar crystallographicpattern of the lattice matching layer 106. FIG. 4 shows a correlationbetween the α-β phase transition temperature and the atomic percentageof Zr and Ti in accordance with an exemplary embodiment. For example,when P_(Zr) is 50%, P_(Ti)=1-P_(Zr)=50%, the α-β phase transitiontemperature is about 605° C. When P_(Zr) is 84%, an example that hasbeen illustrated above, the α-β phase transition temperature may beabout 780° C. In an application of fabricating multi-quantum-well (MQW)devices on the epitaxial layer 104 (e.g. ultra high brightness LEDs),the epitaxial deposition method, such as metal organic chemical vapordeposition and atomic layer deposition and/or any other suitable methodsfor epitaxial growth, may be performed in a temperature range of 700°C.-850° C. In this embodiment, the miltilayer substrate structure may beheated by any heating methods/heat sources under the α-β phasetransition temperature 780° C. but greater than 700° C. in an attempt totransfer the crystallographic pattern of the lattice matching layer 106to the epitaxial layer 104 in a phase, avoiding β phase transition. Thenthe temperature for heating the miltilayer substrate structure duringsubsequent MQW growth, along with any additional device layers, can beraised above or lowered below 780° C. since epitaxial layer 104 has beenformed and is permanently set in the a phase. The temperature may beinitially raised above the α-β phase transition and then immediatelydropped below α-β phase transition temperature to invoke generatingphase transition free energy to crystallize large lateral areasresulting in single crystal α-phase in the lattice matching layer.

By introducing the lattice matching layer 106, the stresses may belowered that might otherwise occur in the epitaxial layer 104 developedduring the epitaxy growth as a result of difference in lattice constantsbetween the substrate 102 and the epitaxial layer 104, and by doing so,aids in the growth of a high crystalline quality epitaxial layer 104. Ifsuch stress is not relieved by the lattice matching layer, the stressmay cause defects in the crystalline structure of the epitaxial layer104. Defects in the crystalline structure of the epitaxial layer 104, inturn, would make it difficult to achieve a high quality crystallinestructure in epitaxy for any subsequent device growth.

As described above, the substrate 102 may comprise a semiconductormaterial, a compound semiconductor material, or another type of materialsuch as a metal or a non-metal. In some embodiments, the substrate 102may be in the form of a polycrystalline solid. Polycrystallinesubstrates may negatively impact the lattice matching layer 106 bymaking it polycrystalline instead of single crystal, thus enlarging thedifference of lattice constants between the lattice matching layer 106and the epitaxial layer 104 (an average lattice constant over multiplegrains and multiple crystalline orientations), and causing extendeddefects such as threading dislocations or grain boundaries, leading topoor crystalline quality of the epitaxial layer 104. To reduce oreliminate the negative impact of a polycrystalline substrate, anamorphous layer 108 may be introduced between the polycrystallinesubstrate 102 and the lattice matching layer 106, as shown in FIG. 1B.By adding the amorphous layer 108 between the polycrystalline substrate102 and the lattice matching layer 106, the impact of thepolycrystalline substrate 102 on the lattice matching layer 106 may bereduced. In this way, only crystallography of the lattice matching layer106 is transferred to the epitaxial layer 104. The amorphous layer 108may comprise, but not limited to, one of silicon dioxide, siliconnitride, tantalum nitride, boronitride, tungsten nitride, glassyamorphous carbon, silicate glass (e.g., borophosphosilicate glass andphosphosilicate glass) and/or other suitable materials. The amorphouslayer 108 may have a thickness of 5 nm to 100 nm.

In some embodiments, the coefficient of thermal expansion of thesubstrate 102 may be different than that of the above layers, resultingin large substrate curvatures. For example, when the coefficient ofthermal expansion of the substrate 102 is greater than that of the abovelayers, biaxial compressive strain arises (e.g. when the substratecomprises sapphire). When the coefficient of thermal expansion of thesubstrate is less than that of the above layers, tensile strain arises(e.g. when the substrate comprises silicon). To overcome the drawbackcaused by the mismatch in the coefficient of thermal expansion, thesubstrate may be used as a thermal matching layer 102 a (shown in FIGS.1C and 1D) by including some chemical elements to accommodate thethermal mismatch between the substrate (namely, the thermal matchinglayer 102 a in this embodiment) and the lattice matching layer 106 asshown in FIG. 1C, or between the substrate and the amorphous layer 108as shown in FIG. 1D. In one embodiment, the thermal matching layer 102 amay comprise molybdenum or its related alloys. The coefficient ofthermal expansion of molybdenum is about 5.4×10⁻⁶/K which isapproximately equal to that of some group III-V compound semiconductors,such as GaN. The substrate may be fabricated using a variety of methodsfor growth of metals, crystals, and their alloys. Example may includeCzochralski, float zone (FZ), directional solidification (DS), zone meltrecrystallization (ZMR), sintering, isostatic pressing, electro-chemicalplating, plasma torch deposition, and/or other suitable methods. Thethermal matching layer 102 a has a thickness in a range of 5 nanometerto 1 millimeter. The thermal matching layer 102 a is also disclosed inU.S. patent application entitled “A Multilayer Substrate Structure.”

By introducing the thermal matching layer and the lattice matchinglayer, the strain caused by the thermal expansion mismatch and latticemismatch may be reduced or completely eliminated. As a result, thedislocation density may be less than 10²/cm² (<100 dislocations persquare centimeter) in the resulting epitaxial layer 104. In developmentof light emitting diodes (LEDs), the reduction or elimination of thestrain may fulfill requirements to overcome the so-called “green gap.”The “green gap” is an industry expression for a droop or decrease in LEDlight output from MQW LEDs that alloy indium with GaN to fabricate greenLED's. This droop in green light outputted occurs for forwardcurrents >50 mA in 1 to 5 square millimeter device areas due to defectdensity resulting from excessive strain from substrates, stress inducedextended defects and point defects propagating into active MQW devicelayers.

Because the human eye is most sensitive to green and green lightstrongly affects the human perception to the quality of white light, thepresent embodiment enables high crystalline quality devices grown onlayer 104. Moreover, exemplary embodiments of the present inventionqualify a cost effective manner of manufacturing a green LED crystallinetemplate. As such, the fulfilling of the “green gap” may enhance thehigh performance of white light emitting diodes based on mixing lightfrom red, green and blue, having the highest theoretical efficacies overphosphor based down conversion LEDs used today.

Many modifications and other example embodiments set forth herein willbring to mind to the reader knowledgeable in the technical field towhich these example embodiments pertain to having the benefit of theteachings presented in the foregoing descriptions and the associateddrawings. Therefore, it is to be understood that the embodiments are notto be limited to the specific ones disclosed and that modifications andother embodiments are intended to be included within the scope of theclaims. Moreover, although the foregoing descriptions and the associateddrawings describe example embodiments in the context of certain examplecombinations of elements and/or functions, it should be appreciated thatdifferent combinations of elements and/or functions may be provided byalternative embodiments without departing from the scope of the appendedclaims. In this regard, for example, different combinations of elementsand/or functions other than those explicitly described above are alsocontemplated as may be set forth in some of the appended claims.

What is claimed is:
 1. A lattice matching layer for use in a multilayersubstrate structure, the lattice matching layer including: a firstchemical element, the first chemical element having a hexagonalclose-packed structure at room temperature that transforms to abody-centered cubic structure at an α-β phase transition temperaturehigher than the room temperature, the hexagonal close-packed structureof the first chemical element having a first lattice parameter; and asecond chemical element, the second chemical element having a hexagonalclose-packed structure at room temperature with similar chemicalproperties to the first chemical element, the hexagonal close-packedstructure of the second chemical element having a second latticeparameter, the second chemical element being miscible with the firstchemical element to form an alloy with a hexagonal close-packedstructure at the room temperature, wherein a lattice constant of thealloy is approximately equal to a lattice constant of a member of groupIII-V compound semiconductors.
 2. The lattice matching layer of claim 1,wherein a linear relation exists between the first and second chemicalelements and their associated lattice parameters at constant temperatureto allow lattice constant of the alloy approximately equal to that ofthe member of group III-V compound semiconductors.
 3. The latticematching layer of claim 1, wherein at least one of the first and thesecond chemical elements belongs to group four elements in periodictable.
 4. The lattice matching layer of claim 1, wherein mole fractionof atomic percentage of the first chemical element to the secondchemical element is about 14% to 86%.
 5. The lattice matching layer ofclaim 1, wherein mole fraction of atomic percentage of the firstchemical element to the second chemical element is about 43% to 57%. 6.The lattice matching layer of claim 1, wherein mole fraction of atomicpercentage of the first chemical element to the second chemical elementis about 99% to 1%.
 7. The lattice matching layer of claim 1 furthercomprising a third chemical element, wherein the third chemical elementhas a hexagonal close-packed structure at the room temperature withsimilar chemical properties to the first and second chemical elements,the hexagonal close-packed structure of the third chemical elementhaving a third lattice parameter, the third chemical element beingmiscible with the first and second chemical elements to form the alloy,a linear relation exists between the first, second and third chemicalelements and their associated lattice parameters at constant temperatureto allow lattice constant of the alloy approximately equal to that ofthe member of group III-V compound semiconductors.
 8. The latticematching layer of claim 1, wherein the coefficient of thermal expansionof the alloy along a plane is approximately equal to that of the memberof group III-V compound semiconductors along the same plane.
 9. Thelattice matching layer of claim 1, wherein the group III-V compoundsemiconductor comprises one of aluminum nitride (AlN), gallium nitride(GaN), indium gallium nitride (InGaN) and indium nitride (InN).
 10. Thelattice matching layer of claim 1, wherein the lattice matching layerhas a thickness in a range of 5 nm to 50 nm.
 11. A method of fabricatinga lattice matching layer, comprising: a first chemical element, thefirst chemical element having a hexagonal close-packed structure at roomtemperature which transforms to a body-centered cubic structure at anα-β phase transition temperature higher than the room temperature, thehexagonal close-packed structure of the first chemical element having afirst lattice parameter; and a second chemical element, the secondchemical element having a hexagonal close-packed structure at roomtemperature with similar chemical properties to the first chemicalelement, the hexagonal close-packed structure of the second chemicalelement having a second lattice parameter, the second chemical elementbeing miscible with the first chemical element to form an alloy withhexagonal close-packed structure at room temperature, wherein latticeconstant of the alloy is approximately equal to lattice constant of amember of group III-V compound semiconductors.
 12. The method of claim11 further comprising interpolating the coefficient of thermal expansionof the first chemical element and the coefficient of thermal expansionof the second chemical element along a plane to arrive at a first orderestimate of the coefficient of thermal expansion of the alloy along thesame plane, wherein the coefficient of thermal expansion of the alloy isabout that of the member of group III-V compound semiconductors alongthe same plane.
 13. The method of claim 11 further comprising heatingthe lattice matching layer below the α-β phase transition temperature toallow epitaxial growth of an epitaxy layer on the lattice matching layerin the a phase.
 14. The method of claim 13 further comprising heatingthe lattice matching layer above the α-β phase transition temperatureand immediately dropping below α-β phase transition temperature toinvoke generating phase transition free energy to crystallize largelateral areas resulting in a single crystal a-phase in the latticematching layer.
 15. The method of claim 11 further comprising growingthe lattice matching layer by one of vacuum evaporation, sputtering,molecular beam epitaxy and pulsed laser deposition, atmospheric chemicalvapor deposition, and atomic layer deposition.
 16. The method of claim11 further comprising growing the lattice matching layer on a substrateand fabricating the substrate using one of Czochralski, float zone (FZ),directional solidification (DS) zone melt recrystallization (ZMR),sintering, isostatic pressing, electro-chemical plating and plasma torchdeposition.
 17. The method of claim 11 further comprising growing thelattice matching layer on an amorphous layer, wherein the amorphouslayer comprises one of silicon dioxide, tantalum nitride, boronitride,tungsten nitride, silicon nitride, glassy amorphous carbon and silicateglass.
 18. A lattice matching layer for use in a multilayer substratestructure comprising a lattice matching layer, the lattice matchinglayer including: a first chemical element, the first chemical elementhaving a hexagonal close-packed structure at room temperature, thehexagonal close-packed structure of the first chemical element having afirst lattice parameter; and a second chemical element, the secondchemical element having a hexagonal close-packed structure at roomtemperature with similar chemical properties to the first chemicalelement, the hexagonal close-packed structure of the second chemicalelement having a second lattice parameter, the second chemical elementbeing miscible with the first chemical element to form an alloy withhexagonal close-packed structure at room temperature, wherein a linearrelation exists between the first and second chemical elements and theirassociated lattice parameters at constant temperature to allow latticeconstant of the alloy approximately equal to that of a member of groupIII-V compound semiconductor.